The present invention relates to an apparatus for optically recording and reproducing data to and from a recording medium and, more particularly, to an apparatus for positioning an optical head on a destination track of the recording medium.
A conventional apparatus for optically recording and reproducing data comprises an optical head for irradiating and receiving a light beam to and from a recording medium on which a plurality of tracks are formed. The position of the optical head is controlled by a head positioning control device which carries out coarse positioning servo control for moving the optical head across the tracks toward a destination track and then carries out fine positioning servo control for adjusting the position of a light beam spot on the destination track by means of an objective lens.
In order to improve the positioning accuracy of the coarse positioning servo control, a number of tracks traversed or crossed by the light beam spot is counted to determine the current position of the optical head so as to produce a track traversing signal. In such a case, the moving speed of the optical head relative to the recording medium is determined by frequency-voltage-converting of the track traversing signal so as to control the driving speed of the optical head.
In the system where the traversing speed is detected until the fine positioning operation of the beam spot to the destination track is carried out, the velocity of the optical head is determined by the frequency of the track traversing signal, and the polarity of the traversing speed signal, i.e., the moving direction of the optical head is determined by a direction command indicative of the traversing direction of the optical head in order to discriminate between alternate track lands and track grooves.
However, in the above described conventional apparatus, a drawback exists in that the accurate direction of the speed signal cannot be obtained while the optical head moves at low speed, i.e., immediately before the fine positioning servo control. In order to stably and promptly change from the coarse to fine servo control, highly accurate speed and accurate detection of the movement of the optical head are needed. However, since the direction of the movement of the optical head, i.e., the direction of the beam spot movement, cannot be correctly detected in the conventional apparatus, another drawback exists in that the optical head runs out of control when the polarity of the voltage speed signal is incorrectly recognized due to external disturbance or eccentricity of the destination track. Further, since discrimination between the track land and the track groove is difficult, a further drawback exists in that the positioning of the destination track may be carried out in a reverse direction, resulting in an unstable fine positioning servo control operation.